1. Field of the Invention
The present invention relates to a scanning electron microscope for semiconductor inspection that detects a scanning image of a sample (semiconductor wafer) by detecting secondary electrons emitted from the sample by irradiation of electron beams (primary electron beams), and in particular, relates to an electron microscope that reduces an influence of foreign bodies attached to the sample.
2. Description of the Related Art
In a scanning electron microscope for semiconductor inspection, an operation of carrying a sample into a vacuum sample chamber and, after the sample being observed, of carrying the sample out of the vacuum sample chamber is performed. An objective lens is excited to cause electron beams to be focused on a sample during observation of the sample and, when observation of the sample is completed, an operation of carrying in the next non-inspected sample is performed. Since an excited state of the objective lens is maintained at this time, the electron microscope is always used in a state in which a magnetic field is generated in the objective lens. The electron microscope is frequently used in a low acceleration voltage area and a technology described in Japanese Patent Application Laid-Open No. 2006-93161 is available as a means for increasing resolution also in the low acceleration voltage area. That is, an electrode is provided in an objective lens part and resolution of secondary electron images is increased by applying a positive high voltage to the electrode. Also in this case, an electric field is applied during observation of the sample and when the sample is carried in and carried out, and the electron microscope is always used while an electric field is generated.
As circuit patterns formed on a semiconductor wafer become increasingly more microscopic and complex, on the other hand, management of foreign bodies of microscopic size has become a big problem because foreign bodies of more microscopic size adhering to the sample surface during observation of the sample seriously affect semiconductor elements.
Factors of microscopic foreign bodies generated in an apparatus of an electron microscope include, in addition to rubbing and abrasion of a sample transfer system mechanism, foreign bodies adhering to the sample surface being carried into the apparatus. Such microscopic foreign bodies are always exposed to a magnetic field of the excited objective lens and a strong electric field by a high voltage applied to the electrode.
For the purpose of reducing foreign bodies generated inside the apparatus, components worn out in the transfer system mechanism are wiped off with a dustproof cloth, cleaned using a lubricant, or replaced. A method disclosed in Japanese Patent Application Laid-Open No. 2006-140070 is known as a method of more actively removing foreign bodies adhering to the surface of a sample. A foreign body removing apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-140070 removes foreign bodies by providing a nozzle-shaped structure in a sample replacement apparatus and injecting a high-pressure gas such as nitrogen toward the surface of a sample.
Japanese Patent Application Laid-Open No. 11-135052 describes, in a scanning electron microscope equipped with a secondary electron detector and an X-ray detector, using an objective lens by switching excitation conditions in order to perform an X-ray analysis without losing a target observation visual field after making a high-resolution observation of a sample.
In a conventional electron microscope, an objective lens is excited for observation of a sample to cause electron beams (primary electron beams) to be focused on the sample and, after observation of the sample, the sample is carried out and the next non-inspected sample is carried in while the excited state of the objective lens is maintained. Since, at this time, a magnetic field generated by an objective lens and a strong electric field generated by an electrode plate act on microscopic foreign bodies adhering to the surface of the sample, a force of attraction acts on such foreign bodies, leading to adhesion to the objective lens and the electrode plate. Then, foreign bodies adhering to the objective lens and the electrode plate accumulate and, when a certain amount is exceeded, such foreign bodies drop onto the surface of the sample and re-adhere as foreign bodies, posing a problem.
As described above, components worn out in a transfer system mechanism are cleaned or replaced as countermeasures to reduce foreign bodies from the viewpoint of preventive maintenance. However, this method requires stopping operation of the apparatus for a long time to perform cleaning or component replacement, creating a problem of a reduced working ratio of the apparatus. Moreover, foreign bodies generated unexpectedly cannot be dealt with and therefore, work will realistically continue in most cases with foreign bodies, posing a new problem.
According to a technique disclosed in Japanese Patent Application Laid-Open No. 2006-140070, a nozzle-shaped structure is provided in a sample replacement chamber and a high-pressure gas such as nitrogen is injected toward the surface of a sample immediately before observation of the sample to remove foreign bodies adhering to the surface of the sample and constituting an obstacle to sample observation by blowing off such foreign bodies. However, there are problems that arrangement of the nozzle-shaped structure between an objective lens of the sample replacement chamber of an electron microscope and a sample is structurally complex and difficult and the degree of vacuum in the sample chamber falls after injecting a gas sufficiently to blow off foreign bodies and thus, a time is needed to increase the degree of vacuum again.
The distance between an objective lens and a sample in a recent electron microscope actually measuring patterns whose dimensions are below 90 nm is only several mm or so, and it is extremely difficult to arrange a nozzle and a pipe for connecting the nozzle in this area. In addition, in order to blow off foreign bodies whose size is 100 nm or larger, a viscous flow of at least several tens to 100 Pa is needed, instead of a molecular flow, but if the degree of vacuum is lowered to such an extent, it becomes difficult for the electron microscope to observe secondary electron images. Thus, after removing foreign bodies by nozzle injecting, it is necessary to exhaust air up to a vacuum region (10−2 to 10−3 Pa) required for observation of SEM images (secondary electron images).
Further, a technology disclosed by Japanese Patent Application Laid-Open No. 11-135052 does not refer to removal of foreign bodies at all.
An object of the present invention is to provide an apparatus that reduces an influence of foreign bodies to allow stable sample observation by preventing foreign bodies attracted to a magnetic field of an objective lens or an electric field of an electrode plate and adhering to the surface of the object lens or the electrode plate from dropping onto the surface of a sample and adhering to the sample during observation of the sample and to provide an apparatus that allows stable sample observation without reducing a working ratio or throughput of the apparatus.